It is known to use down-hole turbines in drill strings in tandem with drill bits. A performance turbine is used in drilling operations to provide higher power and rotational speed to the drill bit. The turbine comprises a turbine body and a turbine shaft, the turbine shaft rotation being independent from the turbine body. The turbine body is connected to the drill string rotating at the drill string speed. The turbine stabiliser may be mounted on a turbine body surrounding the turbine shaft. The turbine body may comprise the turbine stabiliser. The turbine shaft is driven by mud flowing through the turbine and rotates at a higher rotational speed independently of the drill string.
As described above, the performance turbine is connected to the drill string by the turbine body and to the drill bit by the turbine shaft. Mud pumped through the turbine increases the overall rotational speed of the turbine shaft and drill bit. Typically, drill strings strictly powered by rotary tables or top drives rotate at speeds between 50 and 200 rpm while performance turbines can rotate at speeds between 700 and 1800 rpm. This means that currently the available maximum rotational speed of a drill bit connected to the drill string by way of such a performance turbine is around 2000 rpm, that is, the sum of the rotational speed of the rotary table or top drive and the maximum rotational speed of the performance turbine.
In such tandem turbine-drill bit assemblies, the drill bit is connected to the turbine shaft of the performance turbine by means of through a threaded connection. Typically, such a threaded connection comprises a so-called “box” connector (or connection) or female threaded connection provided on the drill bit and a so-called “pin” connector (or connection) or male threaded connection provided on the turbine shaft lower end. Alternatively, the turbine shaft may have the “box” connector or connection and the drill bit may have the “pin” connector or connection. Such threaded connections are determined in accordance with American Petroleum Institute (API) standards.
In typical tandem turbine-drill bit assemblies, the drill bit tends to have a box API connector or connection for engaging with the turbine shaft having a pin API connector or connection. Breaker-slots are located in a suitable position along the gauge or length of the sleeve and extend into the shank. The breaker-slots are provided on the shank of the drill bit so that a tool, known as a bit breaker, can be inserted into the breaker-slots to provide relative rotation, in one direction, between the turbine shaft and the drill bit to make the connection forming the tandem assembly. Similarly, relative rotation in the opposite direction enables removal of the drill bit when it needs to be replaced, for example.
The drill bits used in these tandem assemblies tend typically to be box connection long gauge bits. The term “long gauge bit” refers to a drill bit having a drill head, a gauge, having a length of 125 mm (5″) or longer, and usually with a sleeve having, typically, a length of 150 mm (6″) or longer. The term “short gauge bit” refers to a drill bit having a drill head and a gauge which has a length that is less than 125 mm (5″), usually without a sleeve. As is well understood in the art, the term “drill bit” refers to a cutting head mounted on, or integrally formed with, the shank with or without a sleeve fixed on the shank. Typically, when a bit and turbine are assembled, a gap of between 125 mm to 250 mm (5″ to 10″) remains between the turbine stabiliser and the bit gauge end or the bit sleeve end.
As mentioned above, due to the types of connections provided on bit shanks and turbine shafts, when assembled in tandem, there may be a substantial gap between the turbine stabiliser and the gauge end or sleeve end of the bit. Such a gap allows rings of rock debris and mud resulting from the drilling operation, to form around the turbine shaft and bit shank thereby increasing wear and erosion of the bit shank and/or the turbine shaft. Another drawback of the gap results in a longer length of the turbine-drill bit assembly with reduced rigidity of the drill string. This longer length may also create an instability of the drill bit and vibrations that effectively reduce the drilling efficiency. This is particularly a problem in eccentric turbine-drill bit assemblies or in directional drilling.
By the term “eccentric” is meant that the bit does not have an effective rotational axis that is coincident with the longitudinal axis of the turbine body or drill string, and the effective rotational axis forms a circle around the drill string longitudinal axis. In these conditions, the well diameter drilled is larger than drill bit diameter.